Interior (IBGP) Versus Exterior (EBGP)

Internal BGP neighbors are in the same autonomous system; external BGP neighbors are in different autonomous systems. In general, external neighbors are adjacent to each other and share a common subnet, whereas internal neighbors may be anywhere in the same autonomous system. Figures D-2, D-3, and D-4 illustrate both an External BGP peering session and an Internal BGP peering session between two peers.

Figure D-2. External BGP Session Between Two BGP Peers

Figure D-3. Internal BGP Session Between Two Peers

Figure D-4. Internal (IBGP) and External (EBGP) Peering Sessions

The following figure illustrates routers exchanging both Internal and External BGP sessions.